AUSTIN - Authorities are investigating the cause of a fire that burned a vacant building in southeast Austin early Tuesday morning.

Austin firefighters have been on the scene of the fire near Pleasant Valley and Riverside Drive since around 3:20 a.m. Tuesday. Austin Energy did turn off the power to the buildings near the vacant Champion Sports, and as of 6 a.m. power had not been restored.

The crews will wait until after sunrise to enter the building because the building’s walls were being held up by the roof. Those crews will also work to determine if anyone was inside at the time of the fire.

“There was initial reports of someone inside but that has not been confirmed. Our witness to that has changed his story several times. We're concerned still that there might be someone inside but we're not going to be able to get through in there until after daylight due to the fact that the structure is so unstable,” said Palmer Buck with AFD.

Buck added that the cause of the fire may not be determined, as the scene will be compromised when the walls are knocked down.

Pleasant Valley Road was closed while firefighters were on scene, but lanes have reopened.

This is a developing story. Please check back to KVUE.com for updates.